I had that same issue when I was booked in J DXB-SIN, but once I switched to F it allowed me to select my seat. The only F DXB-SIN flight that worked with my schedule was on the A380 which is great, but it's overnight service which means it's reduced service, i.e., the menu is only breakfast and snacks. Now that I've found space (which wasn't there a few days ago) for the return DXB-SFO in F on my birthday, I may switch to that instead, starting somewhere in Asia that has a feeder in F.